The nicotinamide moiety of the nicotinamide coenzymes (NAD.sup.+ and NADP.sup.+) is derived, in humans, from dietary nicotinamide, nicotinic acid, or the essential amino acids tryptophan or aspartate. Quinolinate is an intermediary in this pathway. L-aspartate oxidase is one of the two components of the quinolinate synthase complex, which is responsible for the conversion of L-asparate and dihydroxyacetone phosphate into quinolinate. L-aspartate oxidase (EC 1.4.3.16) catalyzes the FAD-dependent oxidation of L-aspartate to iminoaspartate, the intermediate that leads to the formation of quinolinate.
Quinolinic acid is a neurotoxin and has been shown to be present at high levels in the central nervous system of patients with certain diseases, such as AIDS, epilepsy, and meningitis. The enzyme quinolinic acid phosphoribosyltransferase provides the only route for quinolinic acid metabolism and is also an essential step in de novo NAD biosynthesis. Quinolinic acid phosphoribosyltransferase (EC 2.4.2.19) is referred to as nicotinate-nucleotide phosphorylase (carboxylating), or quinolinate phosphoribosyltransferase (decarboxylating). This enzyme catalyzes the synthesis of nicotinic acid mononucleotide from quinolinic acid and 5-phosphoribosyl-1-pyrophosphate. The seven-stranded alpha/beta-barrel domain of quinolinic acid phosphoribosyltransferase is very similar in structure to the eight-stranded alpha/beta-barrel enzymes. The structure shows a phosphate-binding site that appears to be conserved among many alpha/beta-barrel enzymes including indole-3-glycerol phosphate synthase and flavocytochrome b2 (Eads, J. C. et al. (1997) Structure 5:47-58). Nicotinate-nucleotide phosphorylase from human liver and brain has been cloned, sequenced and analyzed (Okuno, E. et al. (1988) J. Biochem. (Tokyo) 103:1054-1059.).
Nicotinate-nucleotide phosphorylase and L-aspartate oxidase have been widely studied in prokaryotes and in mammals but they have not yet been described in plants.